1. Field of Use
The present disclosure relates generally to an integrated circuit temperature sensor, and more specifically, to a temperature sensor implementation for isolated or triple well diode structures.
2. Related Art
Low cost and small size make semiconductor devices appealing components for use as temperature sensors. In the case of diodes or diode-connected bipolar junction transistor (BJT) devices, for example, a known voltage drop across the devices at a given temperature allow the devices to be incorporated into a number of temperature sensors.
In many temperature sensors, the sensor component is formed as a stack of two or more diodes or diode-connected BJTs, where the voltage drop across each diode or transistor is sensitive to temperature in a predictable manner. That variable voltage can be measured and translated into a corresponding sensed temperature. FIG. 1, for example, shows an ideal temperature sensor circuit formed using two diodes in a stacked configuration. As shown in FIG. 1, current source 10 supplies a known current Id into a first diode 12. The same current Id passes through diode 12 and enters diode 14, from which the current flows to ground. The circuit includes a node 16 from which a voltage (Vtemp) is measured.
The voltage drop across each of diodes 12 and 14 will vary based upon temperature as the forward voltage of each diode changes in response to ambient temperature. As a result, for a given temperature, the voltage drop across each of the ideal diodes 12 and 14 will be equal and at a predictable value. As a result, the voltage measured at node 16 will also vary predictably based upon the temperature of each of diodes 12 and 14. That voltage can then be measured and used to calculate the temperature of the sensor circuit. In some cases, to improve the resolution of the temperature sensor, additional diodes (e.g., three or more) may be connected in series.
In some real-world implementations of temperature sensors, such as those fabricated in isolated or triple wells in an integrated circuit, the diodes actually operate as part of a parasitic bipolar transistor device. As a result, the transistors actually amplify the current flowing through the temperature sensor by some amount. That amplification affects the device's measured voltage at node 16 resulting in inaccurate temperature readings.